mossback

Origin

From moss + back (in senses 1 (“person with old-fashioned views; one who is very conservative or reactionary”) and 2.3 (“fish that is large and old”) probably a variant of mossyback, which is attested earlier), alluding to someone or something that is so old or has remained so unmoving that it is as if moss has grown on them or it. Sense 2.4 (“largemouth bass”) may refer to the back of the fish being a dark green colour.